In recent years, as high-speed wireless access systems using a 2.4 GHz band or a 5 GHz band, spread of the IEEE (the Institute of Electrical and Electronics Engineers) 802.11g standard, the IEEE 802.11a standard, and so on is remarkable. In these systems, a maximum transmission rate of 54 Mbps (bits per second) in a physical layer has been achieved using an orthogonal frequency-division multiplexing (OFDM) modulation scheme, which is a technology for stabilizing the property in multipath fading environments. It is to be noted that the transmission rate referred to here is a transmission rate on the physical layer, and the actual transmission efficiency in a medium access control (MAC) layer is approximately a little less than 70%. As a result, an upper limit of the actual throughput is approximately 30 Mbps. Moreover, since the transmission efficiency of the MAC layer is further reduced as the number of communication parties which require information is increased, the throughput is also reduced.
On the other hand, in wired local area networks (LANs), a fiber-to-the-home (FTTH) using optical fibers, including a 100 Base-T interface of the Ethernet (registered trademark), is spreading to each individual home. In this way, provision of high-speed lines of 100 Mbps is spreading, and a further increase in the transmission rate is required even in the world of the wireless LANs.
As a technology for increasing the transmission rate in the wireless LANs, the IEEE 802.11n has introduced a multiple-input multiple-output (MIMO) technology, which is a spatial-multiplexing transmission technology. Moreover, in the IEEE 802.11 ac, a multi-user MIMO (MU-MIMO) transmission method, which performs spatial multiplexing for a plurality of users, is being studied (Non-Patent Document 1).
In order to efficiently utilize multi-user MIMO communications in wireless LANs, which are autonomous distributed systems, indication of a group ID using header information appended to the head of a wireless frame is being studied in the IEEE 802.11ac. In the IEEE 802.11ac, this group ID is included in a field called a very high throughput (VHT) signal.
FIG. 21 is a diagram illustrating an example of a configuration of a conventional communication system. The illustrated communication system is provided with an access point 50 and a plurality of stations 60-1 to 60-K (hereinafter, they are simply denoted as “station(s) 60” with respect to common matters for the stations). The access point 50 is provided with a data selection/output unit 5-1, a transmission signal generation unit 5-2, a wireless signal transmission/reception unit 5-3, transmission/reception antennas 5-4-1 to 5-4-N, a reception signal demodulation unit 5-5, a channel information generation unit 5-6, a group-ID table storage unit 5-7, and a group-ID control unit 5-8. Each station 60 is provided with transmission/reception antennas 6-1-1 to 6-1-Mi (i is an integer in a range from 1 to K), a wireless signal transmission/reception unit 6-2, a reception signal demodulation unit 6-3, a transmission signal generation unit 6-4, a group-ID determination unit 6-5, and a group-ID list storage unit 6-6. Here, K denotes the number of the stations 60, Mi denotes the number of the transmission/reception antennas of an i-th station, and N denotes the number of the transmission/reception antennas of the access point 50.
Transmission from the access point 50 to the stations 60 will be considered. Upon input of a transmission data group for one or more stations 60 to the access point 50, the data selection/output unit 5-1 selects data to be transmitted from among the input transmission data group. The data selection/output unit 5-1 then outputs the selected data to the transmission signal generation unit 5-2. Moreover, the data selection/output unit 5-1 outputs information on a set of destination stations 60 to the group-ID control unit 5-8. The group-ID control unit 5-8 refers to a group ID table retained in the group-ID table storage unit 5-7 to acquire a group ID corresponding to the input information on the set of the stations. The group-ID control unit 5-8 outputs the acquired group ID to the transmission signal generation unit 5-2. Here, the group ID table is a table which associates sets of stations 60 with groups to which the sets of stations 60 are currently allocated by the access point 50 among all the group IDs that are available to the access point 50.
The transmission signal generation unit 5-2 performs modulation and coding on the input data, adds control signals such as a pilot signal used for signal detection and a signal for channel estimation, and generates a wireless frame. The transmission signal generation unit 5-2 outputs the generated wireless frame to the wireless signal transmission/reception unit 5-3. At this time, the group ID designated by the group-ID control unit 5-8 is stored in particular bits included in the wireless frame as header information disposed at, for example, the head of the wireless frame. The channel information generation unit 5-6 inputs channel information corresponding to the destination stations 60 of the wireless frame to the transmission signal generation unit 5-2. The transmission signal generation unit 5-2 calculates transmission weights using the channel information and performs a directivity control process of transmission signals. The wireless signal transmission/reception unit 5-3 upconverts the input signal into a carrier frequency and performs transmission through the transmission/reception antennas 5-4-1 to 5-4-N.
The K stations 60-1 to 60-K, which are communication parties, each operate as follows. A wireless signal received through the transmission/reception antennas 6-1-1 to 6-1-Mi is input to the wireless signal transmission/reception unit 6-2. The wireless signal transmission/reception unit 6-2 performs downconversion on a carrier frequency and inputs to the reception signal demodulation unit 6-3. The reception signal demodulation unit 6-3 extracts a received wireless frame using, for example, an autocorrelation process, decodes a signal, and output data.
The group-ID determination unit 6-5 extracts group ID information indicating the destination of a transmission signal from the received wireless frame. Then, the group-ID determination unit 6-5 refers to a group ID list recorded in the group-ID list storage unit 6-6 and determines whether the station 60 itself is a member of the group ID. Here, the group ID list is information indicating whether the station 60 itself is a member of each group ID. If a group ID which does not include the station 60 itself as a member is detected, the group-ID determination unit 6-5 controls the reception signal demodulation unit 6-3 to stop a decoding process of the received signal. As a result, unnecessary decoding operations, i.e., decoding operations of wireless frames that are not destined for the station 60 itself, are eliminated, and the power consumption can be reduced.
If the station 60 itself is a member of the group ID, it is considered that received data is destined for the station 60 itself, and the reception signal demodulation unit 6-3 performs a demodulation operation. Moreover, when the received data includes an instruction for updating the group ID to which the station 60 itself belongs, the group ID list of the station 60 itself retained in the group-ID list storage unit 6-6 is updated.
On the other hand, the following process is performed when transmission from a station 60 to the access point 50 is performed. Once transmission data is generated, the transmission signal generation unit 6-4 performs modulation and coding on the transmission data to generate a signal. Moreover, the transmission signal generation unit 6-4 adds a control signal such as a pilot signal to the generated signal and outputs to the wireless signal transmission/reception unit 6-2. The wireless signal transmission/reception unit 6-2 upconverts an input signal into a carrier frequency and performs transmission through at least one of the transmission/reception antennas 6-1-1 to 6-1-Mi.
In the access point 50, the wireless signal transmission/reception unit 5-3 performs downconversion on a wireless signal received through at least one of the transmission/reception antennas 5-4-1 to 5-4-N and outputs to the reception signal demodulation unit 5-5. Channel information used for demodulation or fed-back channel information between the station 60 and the access point 50 that is included in a demodulated signal is input to the channel information generation unit 5-6. The channel information generation unit 5-6 performs optimization such as calibration on the input channel information to generate transmission weights and outputs to the transmission signal generation unit 5-2.
FIG. 22 is a diagram illustrating an example of a communication environment using group IDs. It is to be noted that in FIG. 22, for convenience of explanation, the access point 50 is denoted as “Y” and the plurality of stations 60 are denoted as letters “A” to “E”. FIG. 23 is a diagram illustrating a group ID table retained in the group-ID table storage unit 5-7 of the access point Y. The group ID table is a table which associates sets of stations with groups to which the sets of stations are currently allocated by the access point Y among all the group IDs that are available to the access point Y.
In the example of FIG. 23, an example in which when the group IDs available to the access point Y are defined using the integers in a range from 0 to 7, group IDs 0 to 4 are allocated to sets of five stations (the station-A, the station-B, the station-C, the station-D, and the station-E), which are in the control of the access point Y, is illustrated. A group ID 7 indicates that communications destined for a single station are performed, and each station is notified of the group IDs 0 to 4 in advance as a group ID list specific to each station. For example, the station-B can recognize that a signal destined for the station-B itself has been transmitted when the group ID is 1 or 3, and the corresponding ranks as a member are second and first, respectively. By referring to these ranks, it is possible to recognize which one of signals that have been subjected to transmission beamforming is destined for the station-B itself Alternatively, it is possible to simply designate only the formation of member stations, instead of including these ranks in information on group IDs.
The access point Y can notify each station of a group ID using part of information included in a wireless frame, e.g., a header portion at the head of the wireless frame. For example, when a wireless frame is to be transmitted to four stations including the station A, the station B, the station C, and the station D, 0 is set therein because it can be seen from the group ID table of FIG. 23 that the corresponding group ID is 0.
A wireless frame includes a header portion and a data portion, and thus it is possible to communicate information on a transmission destination and a transmission source as well as information required for the stations A, B, C, and D to demodulate data. Once a wireless frame has been transmitted, all the stations that are capable of communicating with the access point Y acquire synchronization for the wireless frame and then attempt to demodulate a header portion. The stations A, B, C, D, and E read from the header portion that the group ID is equal to 0. The stations A, B, C, and D each refer to a group ID list retained by each station, determine that a signal is destined for each station itself, and demodulate the subsequent reception signal. It is to be noted that the group ID list retained by each station is notified from the access point Y in advance. In contrast, since it is possible for the station E to determine that the wireless frame is not destined for the station E itself when the group ID that is equal to 0 is read, the station E stops the subsequent reception operation until transmission of the wireless frame is completed.
That is, each station refers to the group ID included in the received wireless frame as to whether each station itself is a member thereof, so that it is possible to determine whether the subsequent reception process is required. As a result, a reduction in the consumed power related to the reception process can be achieved.